Consumer demand for smaller electronic devices has led to various miniaturization efforts in the field of semiconductor manufacturing. In the past decade, along with the increasing demand for the miniaturization in a semiconductor packaging industry, the structure of a package device has evolved from a pin insert type or a through hole mount type to a surface mount type, thereby increasing the mount density for a circuit board. However, increased pin count peripherally designed and arranged around the package device typically results in too short of a pitch of lead wire, yielding limitations of miniaturization on board mounting of the package device.
A chip packaged with wafer level chip scale packaging (WLCSP) technology provides a solution to the above limitations of miniaturization. The WLCSP is a technology in which a whole wafer is packaged and tested first, and then diced into individual chips, instead of the traditional process of assembling the package of each individual unit after wafer dicing. In a typical WLCSP, post-passivation interconnect (PPI) lines, such as redistribution lines (RDLs), are formed with bond pads on metal layers exposed on the surface of the respective chip. Electrical connections are made through bond pads to connect the chip to a package substrate or another die. As a result, the size of a chip after being packaged with the WLCSP is the same as that of the bare chip.
On top of the PPI structures, under-bump metallurgy (UBM) is traditionally formed around the bond pads, and solder balls are individually placed on the bond pads. However, since the formation of the UBM involves the UBM and an additional protection layer (e.g., a second polymer layer), to lower the manufacturing cost, the solder ball in the WLCSP is then directly placed on the bond pad of plain surface of the PPI structure so as to skip the UBM and an additional protection layer from the top surface of the PPI structure.
Nonetheless, in dropping a solder ball to a designated bond pad, the ball shift occurs due to the lack of physical boundary from the UBM to constrain the ball movement, which results in significantly increase in the failure of the reliability tests in the package device.